Organic solvents, electrolytes, and lithium ion cells with good low temperature performance

ABSTRACT

Multi-component organic solvent systems, electrolytes and electrochemical cells characterized by good low temperature performance are provided. In one embodiment, an improved organic solvent system contains a ternary mixture of ethylene carbonate, dimethyl carbonate and diethyl carbonate. In other embodiments, quaternary systems include a fourth component, i.e, an aliphatic ester, an asymmetric alkyl carbonate or a compound of the formula LiOX, where X is R, COOR, or COR, where R is alkyl or fluoroalkyl. Electrolytes based on such organic solvent systems are also provided and contain therein a lithium salt of high ionic mobility, such as LiPF 6 . Reversible electrochemical cells, particularly lithium ion cells, are constructed with the improved electrolytes, and preferably include a carbonaceous anode, an insertion type cathode, and an electrolyte interspersed therebetween.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of U.S. Provisional Application SerialNo. 60/088,125 filed, June 4, 1998, the contents of which are fullyincorporated herein.

ORIGIN OF INVENTION

The invention described herein was made in the performance of work undera NASA contract, and is subject to the provisions of Public Law 96-517(35 USC 202) in which the Contractor has elected to retain title.

FIELD OF THE INVENTION

The present invention is directed to organic solvents and electrolytesfor electrochemical cells, particularly lithium ion cells, and toelectrochemical cells exhibiting good low temperature performance.

BACKGROUND OF THE INVENTION

State-of-the-art lithium ion cells typically include a carbon (e.g.,coke or graphite) anode intercalated with lithium ions to form Li_(x)C;an electrolyte consisting of a lithium salt dissolved in one or moreorganic solvents; and a cathode made of an electrochemically activematerial, typically an insertion compound, such as LiCoO₂. During celldischarge, lithium ions pass from the carbon anode, through theelectrolyte to the cathode, where the ions are taken up with thesimultaneous release of electrical energy. During cell recharge, lithiumions are transferred back to the anode, where they reintercalate intothe carbon matrix.

Lithium ion rechargeable batteries have the demonstrated characteristicsof high energy density, high voltage, and excellent cycle life, makingthem more attractive than competing systems such as Ni—Cd and Ni—H₂batteries. However, few state-of-the-art lithium ion cells perform wellat low temperatures making them unsuitable for many terrestrial andextra-terrestrial applications. Many scheduled NASA missions demand goodlow temperature battery performance—without sacrificing such propertiesas light weight, high specific energy, long cycle life, and moderatecost. The Mars Exploration Program, for example, requires rechargeablebatteries capable of delivering 300 cycles with high specific energy,and the ability to operate over a broad range of temperatures, includingthe extremely low temperatures encountered on and beneath the surface ofMars. Mars Rovers and Landers require batteries that can operate attemperatures as low as −40° C. Mars Penetrators, which will penetratedeep into the Martian surface, require operation at temperatures lowerthan −60° C.

To be used on the Mars missions and in low earth orbit (LEO) andgeostationary earth orbit (GEO) satellites, as well as in terrestrialapplications, lithium ion rechargeable batteries should exhibit highspecific energy (60-80 Wh/Kg) and long cycle life (e.g., <500 cycles).

Unfortunately, state-of-the-art lithium ion cells typically exhibit poorcapacities below 0° C. This is primarily due to limitations of theelectrolyte solutions, which become very viscous and freeze at lowtemperatures, resulting in poor ionic conductivity. In addition, thesurface film (i.e., solid electrolyte interphase, SEI) that forms on theelectrodes either builds up over the course of repeated charge/dischargecycling or becomes highly resistive at lower temperatures. Ideally, theSEI layer on the carbon anode needs to be protective toward electrolytereduction and yet conductive to lithium ions to facilitate lithium ionintercalation, even at low temperature.

A number of factors can influence the low temperature performance oflithium ion cells, including (a) the physical properties of theelectrolyte (i.e., conductivity, melting point, viscosity, etc.), (b)the electrode type, (c) the nature of the SEI layers that can form onthe electrode surfaces, (d) cell design, (e) electrode thickness,separator porosity and separator wetting properties. Of these, theelectrolyte properties have the predominant impact upon low temperatureperformance, as sufficient electrolyte conductivity is a necessarycondition for good performance at low temperatures. Ideally, a good lowtemperature performance electrolyte should possess a combination ofseveral critical properties, including high dielectric constant, lowviscosity, adequate Lewis acid-base coordination behavior, as well asappropriate liquid ranges and salt solubilities in the medium.

Conventional electrolytes employed in state-of-the-art lithium ion cellshave typically consisted of binary mixtures of organic solvents, forexample high proportions of ethylene carbonate, propylene carbonate ordimethyl carbonate, within which is dispersed a lithium salt, such asLiPF₆. Examples include 1.0M LiPF₆ in a 50:50 mixture of ethylenecarbonate/dimethyl carbonate, or ethylene carbonate/diethyl carbonate.Such electrolytes do not perform well at low temperature because theybecome highly viscous and/or freeze.

It can be seen, therefore, that a clear need exists for improved organicsolvents, electrolytes, and electrochemical cells capable of performingwell at low and moderate temperatures, with high specific energy andhigh cycle lives.

SUMMARY OF THE INVENTION

The present invention provides novel organic solvent systems,electrolytes, and electrochemical cells characterized by improved lowtemperature performance, including high conductivity, good cycle life,good discharge characteristics, good stability and self-dischargecharacteristics, and excellent compatibility with the cell components,as well as excellent room temperature and elevated temperatureperformance. Lithium ion cells containing the new electrolytes are idealfor use in portable electronic products, space vehicles, and otherapplications.

In one embodiment of the invention, an organic solvent system comprisesa ternary mixture of ethylene carbonate (EC), dimethyl carbonate (DMC),and diethyl carbonate (DEC), preferably an equal volume mixture of each.Fluorinated analogs—especially perfluorinated analogs—can be used inplace of one or more of the solvents.

In a second embodiment of the invention, an organic solvent systemcomprises a mixture of organic carbonates and at least one aliphaticester, preferably an alkyl or fluoroalkyl ester. Thus, ternary,quaternary, and higher solvent systems are provided. One such solventsystem comprises a ternary mixture of EC, DMC, and MA (methyl acetate).Another solvent system comprises a quaternary or higher mixture of EC,DMC, DEC and at least one alkyl or fluoroalkyl ester.

In still another embodiment, an improved organic solvent system includestwo or more alkyl carbonates, preferably EC, DMC, DEC, and/or PC(propylene carbonate), and an asymmetric alkyl carbonate.

In yet another embodiment, the solvent system includes a compound havingthe formula LiOX (where X is —R, —COOR, or —COR, where R is alkyl orfluoroalkyl) or another basic species that can effectively catalyze thedescribed disproportionation reactions. Fluorinated (especiallyperfluorinated) analogs of one or more of the co-solvents or additivescan also be used. Where the organic solvent system includes anasymmetric alkyl carbonate, such as ethyl methyl carbonate, it can beadded directly to the solvent system or generated in situ by including alithium alkoxide or similar basic species in the mixture. Thus, thepresent invention also provides a method for making organic solventsystems for electrochemical cells in which asymmetric alkyl carbonatescan be produced.

The addition of lithium methoxide or a related basic species also hasthe observed benefit of improving the SEI formation characteristics ofcarbonate-based (and other) electrolytes, which contributes to improvedcell performance, especially at low temperature, due to low electrodepolarization behavior. Lithium methoxide has previously been detected asa by-product formed from electrolyte reactions in lithium-ion cellswhich have been cycled or exposed to lithiated graphite. In fact, theaddition of alkoxides and related basic compounds to lithium ion cellelectrolytes has now been found to have a beneficial effect,irrespective of whether the additive(s) can facilitatedisporportionation/exchange reactions resulting in asymmetric carbonateformation. The present invention, therefore, includes a method of makingan improved electrolyte by adding to an electrolyte solvent system asmall amount of a compound of the formula LiOX (as described herein), ora similar basic species.

In addition to novel organic solvent systems, the invention alsoprovides a variety of improved electrolytes, comprising a lithium salthaving high ionic mobility, dispersed in an organic solvent system. Apreferred salt is LiPF₆. The electrolytes are particularly well-suitedfor use in lithium ion cells, especially where low temperatureperformance is required.

In another embodiment of the invention, an improved, reversibleelectrochemical cell comprises an anode, a cathode, and an electrolyteas described herein. Preferably, the anode and cathode are of theinsertion type, i.e., a coke, graphite, or modified carbon anodeintercalated with lithium ions, and the cathode is a lithiated metaloxide, such as LiCoO₂, or a similar material. In operation of the cell,lithium ions pass from the carbon through the electrolyte to thecathode, where they are taken up. During recharge, lithium ions aretransferred back to the anode, where they reintercalate into the carbonmatrix.

Lithium ion cells prepared in accordance with this invention have highspecific energy, energy density, operating voltage, and coulombicefficiency, and low self-discharge tendencies. Cell performance at lowtemperatures (<−20° C.)is notably improved over comparable,state-of-the-art systems. When used in upcoming space missions, theinvention promises the additional benefits of improved reliability, andbroader operating range.

DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the invention will be betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic, partially exploded view of an electrochemicalcell constructed according to one embodiment of the present invention;and

FIGS. 2-35 are graphs comparing various performance characteristics ofcells containing prior art electrolytes and cells containing improvedelectrolytes according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In designing electrolytes that are highly conductive at lowtemperatures, it is necessary to consider a number of importantparameters, such as the dielectric constant of the medium, electrolyteviscosity, Lewis acid-base coordination behavior, and the appropriatesolvent freezing points and melting points, and solubilities of theelectrolyte salt. To be a viable candidate of lithium-ion cellapplications, the electrolyte solution must satisfy a number ofrequirements in addition to good conductivity over the specifiedtemperature range, namely, (i) good electrochemical stability over awide voltage window (e.g, 0 to 4.5V), (ii) the ability to form thin,stable passivating films at the carbonaceous anode electrode, and (iii)good thermal and chemical stability. The physical properties of a numberof organic carbonates employed in lithium ion cells are shown in Table1.

TABLE 1 Dielectric Viscosity Constant Donor Melting Boiling η ε NumberPoint Point Density Abbr. Organic Carbonate (cP, 25° C.) (25° C.) D_(N)M.W. (° C.) (° C.) (g/ml) EC Ethylene Carbonate 1.85 89.6 16.4 88.06  39243 1.321 (1,3-dioxolane-2- (40° C.) (40° C.) one) PC PropyleneCarbonate 2.53 64.4 15.1 102.09 −55 240 1.189 (1,2-propanediol cycliccarbonate) DMC Dimethyl Carbonate 0.59 3.12 NA 90.08  3  90 1.069 DECDiethyl Carbonate 0.75 2.82 NA 118.13 −43 126 0.975 DPC DipropylCarbonate NA NA NA 146.19 NA 147 0.944 EMC Ethyl Methyl 0.66 2.4 NA104.12 −14 107 1.007 Carbonate MPC Methyl Propyl 0.9 NA NA 118.13 −49130 0.980 Carbonate (19° C.) EPC Ethyl Propyl 0.9 NA NA 132.16 −81 1480.950 Carbonate (19° C.) NA = Not Available

It has now been found that a mixture of organic carbonates, and in someembodiments, other additives, leads to improved low temperatureconductivity and increased compatibility of the electrolyte with theelectrochemical cell. Organic solvent systems and electrolytes based ona ternary or quaternary mixtures of organic carbonates and otheradditives have been found to exhibit good low temperature performanceand cycle life when used in a lithium ion electrochemical cell. Thus, inone embodiment of the invention, an improved organic solvent systemcomprises a mixture of ethylene carbonate, dimethyl carbonate, anddiethyl carbonate, preferably in a 1:1:1 equal volume mixture. (Unlessotherwise noted, the solvent systems disclosed herein are described on avolume-by-volume (v/v) basis.) Organic carbonates such as ethylenecarbonate, dimethyl carbonate, and diethyl carbonate are available fromAldrich Chemical company, Inc.(Milwaukee, Wis.), and Mitsubishi ChemicalCorporation (U.S. distributor, NY, N.Y.).

Particularly good results are seen with Particularly good results areseen with the (1:1:1 v/v) ternary formulation, which improves the lowtemperature performance of lithium-ion cells. This stems from a numberof factors, including the use of a mixed solvent system, low EC content(<50% by volume) necessary for high conductivity at low temperatures,and a sufficient amount of each of DMC (for low viscosity and highstability/compatibility), DEC (for low melting characteristics), and EC(for effective passivation of electrodes and high temperaturestability).

Electrolytes that exhibit good low temperature performance inelectrochemical cells are readily prepared from such ternary systems bydissolving therein a lithium salt having high ionic mobility.Non-limiting examples of such salts include LiPF₆, LiAsF₆, LiSbF₆,LiClO₄, LiCF₃SO₃ and other perfluoroalkyl or -aryl sulfonates, LiCF₃CO₂,LiBF₄, lithium closo-boranes (i.e., LiB₁₀Cl₁₀, LiB₁₂C₁₂), lithiumorganoborates (i.e.,LiB(CH₃)₄, LiB(C₆H₅)₄, etc.) and perfluorinatedalkyl or aryl borates, lithium bis(trifluoromethylsulfonyl)imide, andlithium tris(trifluoromethylsulfonyl) methide. Preferred lithium saltsare those possessing a number of characteristics, including thermal,chemical, and electrochemical stability, high solubility in dipolaraprotic solvents, high conductivity over a range of temperatures, andcompatibility with cell components. Ideally, the salt should benon-toxic and low cost. LiPF₆ is preferred. LiPF₆ and many other lithiumsalts are available from Aldrich, Mitsubishi, and other vendors.

Although the optimum salt concentration depends on several factors,including the effect on melting point, viscosity, conductivity, andsolubility, in general, good ternary and quaternary solvent electrolytesexhibiting good low temperature performance can be prepared with lithiumsalt concentrations ranging from about 0.5 to 1.5M, more preferably,about 0.75 to 1.25M. For obvious reasons, the solvents and electrolytesshould be anhydrous.

In addition to investigating low viscosity additives, we have studiedthe effect of salt type and salt concentration upon the conductivity ofelectrolyte solutions at low temperature. It was determined that anelectrolyte solution consisting of 0.75M LiPF₆ in EC+DEC+DMC (1:1:1)showed an improvement in conductivity at temperatures below −20° C., ascompared to 1.0M LiPF₆ in EC+DEC+DMC (1:1:1). In addition, theelectrolyte formulation 0.5M LiPF₆ in EC+DEC+DMC+MA (1:1:1:1) displayedbetter conductivity at temperatures below −40° C., as compared to the1.0M LiPF₆ in EC+DEC+DMC+MA (1:1:1:1) solution. This electrolytesolution should be ideal for ultra-low temperature applications, due tothe fact that the conductivity was observed to be greater than 1 mS/cmat −60° C. Although not bound by theory, the improvement in lowtemperature conductivity observed as the salt concentration decreases isunderstood to be primarily due to viscosity effects, which increase withincreasing salt concentration. Thus, the benefit of lowering thefreezing point of a mixture by using higher salt concentrations(freezing point depression effect) must be weighed accordingly with theprohibitive viscosity increase that results in some cases. Ultimately,the range of operating temperatures of a given application will dictatethe appropriate salt concentration selection.

In another embodiment of the invention, ternary, quaternary and higherorganic solvent systems and electrolytes for low temperature lithium ioncells comprise a mixture of carbonates and at least one aliphatic ester,preferably an alkyl ester with general formula R′ COOR″, where R′ and R″are, independently, C₁-C₁₀ aliphatic, especially C₁-C₁₀ alkyl, includingbranched, straight chain, and cycloaliphatic. Fluorinated aliphaticanalogs are acceptable alternatives. In one embodiment, an improvedelectrolyte solvent system comprises a mixture of ethylene carbonate,dimethyl carbonate, diethyl carbonate and one or more aliphatic esters,preferably an equal volume (1:1:1:1) mixture. Improved electrolytes aremade with such systems by dissolving therein a lithium salt, forexample, LiPF₆.

Quaternary solvent systems can be tailored to have lower viscosity andfreezing points then the ternary alkyl carbonate mixture, yet, due tothe presence of the alkyl carbonate components, provide desirable filmformation characteristics. Improved low temperature conductivity throughreduced viscosity and freezing points is provided by the quaternaryadditive, i.e., the alkyl ester. In fact, quaternary mixtures appear tobe more desirable for low temperature lithium ion cells, due to the factthat the electrolyte properties (i.e., dielectric constant, viscosity,liquid range, coordination properties, and overall stability) can bemore easily tailored in multi-component systems. In addition, mixedsolvent systems appear to produce more highly conductive solutions atlow temperatures, as compared to single solvent systems, due to adisordering effect in the lithium ion coordination behavior of thesolvent medium.

A number of aliphatic esters are commercially available from AldrichChemical Company including, without limitation, esters such as methylformate (MF), ethyl formate (EF), propyl formate (PF), propyl butyrate(PB), methyl acetate (MA) (m.p.=−98° C., b.p.=57.5° C.), ethyl acetate(EA), propyl acetate (PA), butyl acetate (BA) (m.p.=−78° C.), methylpropionate (C₂H₅CO₂CH₃) (m.p.=−88° C., b.p.=79° C.), ethyl propionate(C₂H₅CO₂C₂H₅), propyl propionate (C₂H₅CO₂C₃H₇) (m.p.=−76° C.), butylpropionate (C₂H₅CO₂C₄H₉), methyl butyrate (C₃H₇CO₂CH₃) (b.p.=102° C.)ethyl butyrate (C₃H₇CO₂C₂H₅), propyl butyrate (C₃H₇CO₂C₃H₇), butylbutyrate (C₃H₇CO₂C₄H₉) (b.p.=165° C.), methyl valerate (C₄H₉CO₂CH₃)(b.p.=128° C.), ethyl valerate (C₄H₉CO₂CH₃) (b.p.=144° C.), ethylheptanoate (C₆H₁₃CO₂C₂H₅) (m.p.=−66° C., b.p.=188-189° C.), methylenanthate (b.p.=172° C.), (C₆H₁₃CO₂CH₃), pentyl propionate(C₂H₅CO₂C₅H₁₁) (b.p.=169° C.),hexyl acetate (CH₃CO₂C₆H₁₃), (m.p.=p−80°C., b.p.=168-170° C.), methyl caproate (C₅H₁₁CO₂CH₃) (m.p.=−71° C.,b.p.=151° C.), methyl caprylate (C₇H₁₅CO₂CH₃) (b.p.=194° C.), amylacetate (CH₃CO₂C₅H₁₁) m.p.=−100° C., b.p.=149° C.), ethyl isovalerate((CH₃)₂CHCH₂CO₂C₂H₅) (m.p.=−99° C., b.p.=131° C.), ethylcaproate(C₅H₁₁CO₂C₂H₅) (b.p.=168° C.), ethyl caprylate (C₇H₁₅CO₂C₂H₅)(m.p.=−47° C., b.p.=206-208° C.), ethyl caprate(C₉H₁₉CO₂C₂H₅) (b.p.=245°C.), neopenyl pivalate ((CH₃)₃CCO₂CH₂C(CH₃)₃) (b.p.=165° C.), methylnonaoate (methyl perlargonate) (C₈H₁₇CO₂CH₃) (b.p.=213-214° C.), octylacetate (CH₃CO₂C₈H₁₇), (b.p.=211° C.), ethyl nonanoate (C₈H₁₇CO₂C₂H₅),methyl decanoate (C₉H₁₉CO₂CH₃), methyl undecanoate (C₁₀H₂₁CO₂CH₃), anddodecyl acetate (CH₃CO₂C₁₂H₂₅) (b.p.=150° C./15 mm). Thephysico-chemical properties of a few of these solvents are listed inTable 2.

TABLE 2 Melting Boiling Point Point Density Viscosity Abbr. Alkyl EsterM.W. (° C.) (° C.) (g/ml) η (cP) MF Methyl 60.05 −100°  34° 0.974 0.325Formate (25° C.) EF Ethyl Formate 74.08 −80° 53° 0.917 0.402 (20° C.) MAMethyl Acetate 74.08 −98°   57.5° 0.932 0.381 (20° C.) EA Ethyl Acetate88.11 −84° 77° 0.902 0.455 (20° C.) MP Methyl 88.11 −88° 79° 0.915 0.431Propionate (25° C.) EP Ethyl 102.13 −73° 99° 0.892 0.564 Propionate (15°C.) MB Methyl 102.13 −85° 102°  0.896 0.541 Butyrate (25° C.  EB EthylButyrate 116.16 −101°  121°  0.879 0.711 (15° C.)

Aliphatic esters possess low freezing points (i.e., about −70 to −100°C.) and viscosities, and are fully miscible in organic carbonatesystems. High molecular weight esters have the advantage of high boilingpoints (>150° C.) and low melting points, and are ideal for applicationswhere good high temperature and low temperature performance arerequired.

In one embodiment of this aspect of the invention, a quaternary mixtureof organic solvents comprises an equal volume (1:1:1:1) mixture of ethylcarbonate, diethyl carbonate, dimethyl carbonate and one or morealiphatic ester, (preferably ethyl propionate or ethyl butyrate). Moregenerally, the amount of quaternary additive (i.e., the ester) can rangefrom about 5 to 50%, on a volume percent basis. Thus, in addition to1:1:1:1 mixtures, other representative formulations includeEC+DEC+DMC+EB (1:1:1:2) and EC+DEC+DMC+EB (1:1:1:3).

Comparative studies between quaternary formulations differing only intheir selection of aliphatic ester display a conductivity trend thatfavors the low molecular weight esters over the higher molecular weightesters. However, the interfacial stability with the graphite electrodeduring cell charge-discharge cycling appears to be adversely affectedsuch that the kinetics of lithium intercalation are more severelyhindered with the low molecular weight esters, as compared with thehigher molecular weight esters, which exhibit adequate interfacialstability and enhanced electrolyte conductivity, resulting in improvedlow temperature performance for lithium ion rechargeable cells.Consequently, quaternary organic solvent systems containing ethylbutyrate or ethyl propionate are preferred over those containing methylor ethyl acetate.

Electrolytes based on the quaternary organic solvent systems are readilyprepared by mixing a lithium salt, preferably LiPF₆, with the organiccarbonates and aliphatic ester(s).

In another embodiment of the invention, an asymmetric carbonate, forexample ethyl methyl carbonate (EMC) or methyl propyl carbonate (MPC),is used to prepare ternary or quaternary organic solvent systems andelectrolytes. Ethyl methyl carbonate has previously been reported to bea viable solvent for lithium ion battery electrolytes based on binaryand ternary solvent formulations. It has now been found that quaternaryformulations containing ethyl methyl carbonate are a substantialimprovement over binary and ternary EMC-containing electrolytes.Although not bound by theory, it is believed that such quaternarysystems provide higher conductivity due to disordered coordinationcomplexes that form therein. Also, the selected quaternary solventexhibits improved low temperature conductivity, due to its lowerfreezing point and viscosity.

Ethyl methyl carbonate is available from FMC Corporation (Chicago,Ill.). It can also be generated in situ by adding a small (essentially,catalytic) amount of lithium methoxide or ethoxide to a mixture ofdimethyl carbonate and diethyl carbonate. Methyl propyl carbonate can begenerated in situ by adding a small amount of lithium methoxide to amixture of dimethyl carbonate and dipropyl carbonate. A particularlypreferred quaternary system based on an asymmetric alkyl carbonatecomprises a 1:1:1:1 volume mixture of ethylene carbonate, dimethylcarbonate, diethyl carbonate, and ethyl methyl carbonate. Addition ofLiPF₆ or another suitable lithium salt, preferably in a concentration offrom about 0.5 to 1.5M, results in an improved electrolyte that exhibitsgood low temperature performance and other properties required oflithium ion cells.

In another aspect of the invention, ternary, quaternary or higherorganic solvent systems and electrolytes are provided, and contain asmall amount of an additive, namely, a lithium compound of the formulaLiOX, where X is R, COOR, or COR, where R is alkyl, fluoroalkyl, orperfluoroalkyl, or a similar basic species. The LiOX additive is astrong base, and is believed to facilitate the formation of an SEIhaving desirable characteristics and minimize the deleterious effects ofacid-promoted degradation, such as the undesirable action of hydrogenfluoride produced during cell operation. In addition to having favorableSEI film formation characteristics, lithium alkoxides and similar basescan be used to initiate base-catalyzed carbonate exchange reactions inmixtures of symmetrical carbonates, yielding mixtures containingasymmetric carbonates. The following examples are representative:

Other examples of asymmetric carbonates include methylpropyl carbonate,butylmethyl carbonate, ethylpropyl carbonate, and butylethyl carbonate,where propyl can be either n-propyl or iso-propyl and butyl can beeither n-butyl, i-butyl, or t-butyl.

It will be appreciated that compounds of the formula LiOR are lithiumalkoxides. Non-limiting examples include lithium methoxide, lithiumethoxide, lithium iso-propoxide, ,and lithium butoxide, all of which areavailable from Aldrich Chemical Company. Compounds of the formulaLiOCOOR are lithium alkoxy esters. Similar compounds, such as lithiumacetoacetate (Aldrich) are also acceptable. Compounds of the formulaLiOCOR are lithium carboxylates. A non-limiting example is lithiumacetate, available from Aldrich. Fluorinated analogs, includingperfluorinated compounds of the formula LiOX (where X is as describedabove) include such compounds as lithium trifluoroacetate (Aldrich.)Other basic species include aromatic compounds, such as lithiumphenoxide, lithium benzoate, and lithium cyclohexanebutyrate (eachavailable from Aldrich); lithium diisopropylamide (LDA) and lithiumdimethylamide (Aldrich); lithium metaborate; lithiumtrimethylsilanolate; and alkyl or aryl lithiums, for example butyllithium, hexyl lithium, and methyl lithium (Aldrich).

Compositions containing a lithium compound of the formula LiOX (or asimilar basic species) are made by adding a small amount of the compoundto the organic solvent mixture or electrolyte. Preferred compositionshave an LiOX concentration of about 0.0001M to 0.1M, more preferablyabout 0.01M.

From the preceeding comments, it will be appreciated that another aspectof the invention is a method of improving electrolyte performance(making an improved electrolyte), particularly carbonate-, ether-, andester-based electrolytes. Thus, an improved electrolyte is made byadding to the solvent mixture, or salt+solvent mixture, a small amountof a basic species, preferably a compound of the formula LiOX, where Xis as described above. Other basic compounds (exemplified above) can beused in the alternative.

In another aspect of the invention, the organic solvents andelectrolytes described herein are used in the construction of animproved electrochemical cell, characterized by good low temperatureperformance. Essentially, the cell comprises an anode, a cathode, and anelectrolyte. Particularly preferred are lithium ion cells, oneembodiment of which is schematically illustrated in FIG. 1. The cell 10has a carbonaceous anode 12 separated from an insertion-type cathode 14by one or more electrolyte-permeable separators 16, with theanode/separator(s)/cathode cylindrically rolled up in “jelly roll”fashion and inserted into a can or case 18, which is sealed or closed bya cap 20. Both the anode and the cathode are bathed in an electrolyte(not shown) as described above, which is able to pass through theseparator(s), allowing ion movement from one electrode to the other.Other features, such as one or more gaskets, anode tabs, safety vents,center pin, and other features known in the art can be included asdeemed appropriate, in accordance with well known battery design andfabrication practice.

Carbon is the preferred anode material for lithium ion rechargeablecells due to its low potential versus lithium (of the lithiatedcompound), excellent reversibility for lithiumintercalation/deintercalation reactions, good electronic conductivity,and low cost. Three broad types of carbonaceous anodic materials areknown: (a) non-graphitic carbon, i.e., petroleum coke, pitch coke (b)graphitic carbon, i.e., natural graphite, synthetic graphite, and (c)modified carbon, i.e., meso-carbon micro-bead (MCMB) carbon material. Apreferred anodic material is KS-44, a synthetic graphite available fromLonza AG (Basel, Switzerland).

Suitable cathode materials include transition metal oxides, especiallyinsertion type metal oxides. Nonlimiting examples include lithiatedcobalt oxides (i.e., LiCoO₂), lithiated nickel oxides (i.e., LiNiO₂),lithiated manganese oxides (i.e., LiMn₂O₄), lithiated vanadium oxides(i.e., LiV₃O₈), and lithiated mixed metal oxides (i.e.,LiCo_(x)Ni_(1-x)O₂). In lithium ion cells, the cathode functions as asource of lithium for the intercalation/deintercalation reactions at theanode and the cathode, because of the instability of carbon materials inthe lithiated state. Also, the cathode material in lithium ion cellsmust have a high voltage versus lithium (>3.0V) to compensate forvoltage losses due to the use of alternate lithium anode materials(having reduced lithium activity) such as lithiated carbon.Consequently, lithium transition metal oxides are preferred over binarytransition metal chalcogenides. The presently preferred intercalationcompounds meeting these requirements are LiCoO₂, LiNiO₂, LiMnO₂, LiM₂O₄and Li₂V₆O₁₃. Lithiated cobalt oxide (LiCoO₂) is the preferred materialbecause of its ease of preparation and reversibility. Lithiated nickeloxide and lithiated manganese oxide are good alternatives.

In order to achieve long life in lithium ion cells, a case-negativedesign is preferred. Case-neutral designs are also acceptable.Case-positive design is not preferable in view of the unstable behaviorof nickel-plated mild steel/stainless steel materials at highervoltages.

As compared to conventional cells (such as Ni—Cd, Ni—MH, Ag—Zn,Pb-acid), lithium ion cells generally require very thin electrodes,typically in the range of 5-10 mils, to achieve a high charge/dischargerate capability to offset the poor lithium ion defusion characteristicsof these electrode materials.

To prevent contact between the anode and cathode, each electrode ispreferably wrapped in a separator comprising a microporous polypropyleneor polyethylene material, for example Cellguard 2500, manufactured byCelanese Corp. (Charlotte, N.C.). Because of cell resistanceconsiderations, inter-electrode distances should be about 2 mil or lessto achieve high rate capability. The preferred thickness of theseparator is about 1 mil. Other factors to be considered in selectingthe separator material are separator porosity, electrolyte wettingproperties, and mechanical properties.

Copper foil is the preferred anode current collector material because ofits low cost and high electronic conductivity. Nickel foil can also beconsidered as an anode current collector. Aluminum foil is the preferredcathode current collector material because of its stability at highvoltages (>4.0V) and low cost.

It will be appreciated that a variety of design modifications can bemade to the cell without departing from the present invention. Forexample, the cell need not be cylindrical, but can be prismatic, withrectangular electrodes stacked in alternating fashion. The overall sizeof the cell is variable; AA up to D-size cells have been prepared inaccordance with the present invention.

Examples and Electrochemical Measurements

To evaluate the electrochemical properties of various embodiments of theinvention, a series of electrolytes based on ternary and quaternarysolvent systems were prepared and tested in half-cell and/or reversiblelithium ion cells. For comparative purposes, a series of electrolytesbased on binary solvent systems, i.e., ethylene carbonate+dimethylcarbonate, ethylene carbonate+diethyl carbonate, propylene carbonate(PC)+diethyl carbonate, and propylene carbonate+dimethoxyethane (DME),was also prepared. Battery-grade purity organic carbonates, containingthe desired concentration of LiPF₆ salt, were purchased from MitsubishiChemicals, and were certified to contain <50 ppm H₂O content. Alkylesters were obtained from Aldrich Chemical Company. Lithium methoxidewas purchased from Aldrich Chemical Company and used as received. Table3 summarizes the examples (denoted “Ex.”) and comparative samples(denoted “C.Ex.”). Solvent amounts are expressed on a relative amount byvolume basis, e.g., 1:1:1, 1:1:1:2, etc. Salt concentration is expressedin units of molarity. In some cases, and as indicated in the notes,graphite-type anodes were used, while in other cases coke-type anodeswere used.

Some samples were evaluated in 150-300 mAh experimental half-cellstudies using O-ring-sealed, glass cells equipped with a “jelly roll”carbon electrode (KS-44 graphite from Lonza) as the cathode and lithiummetal as the anode, and a lithium metal reference electrode.Electrochemical measurements were made using an EG&GPotentiostat/Galvanostat (and Solartron 1255 Frequency Response Analyzerfor impedance) measurements interfaced with an IBM PC, using Softcorr352 (and M388 for impedance) software. Cycling data were collected usingan Arbin battery test system.

TABLE 3 Solvent Sample [LiOCH₃] Solvent System Ratio C. Ex. 1 1.0 EC +DMC (3:7) C. Ex. 2 1.0 EC + DEC (3:7) C. Ex. 3 1.0 PC + DEC (1:1) C. Ex.4 0.5 PC + DEC (2.5:7.5) C. Ex. 5 0.5 PC + DME (1:1) C. Ex. 6 0.75 EC +DMC (3:7) C. Ex. 7 0.75 EC + DMC (3:7) C. Ex. 8 1.0 EC + DMC + EMC(1:1:1) Ex. 1 1.0 EC + DEC + DMC (1:1:1) Ex. 2 1.0 EC + DEC + DMC + MA(1:1:1:1) Ex. 3 1.0 EC + DEC + DMC + EA (1:1:1:1) Ex. 4 1.0 EC + DEC +DMC + EP (1:1:1:1) Ex. 5 1.0 EC + DEC + DMC + EB (1:1:1:1) Ex. 6 0.75EC + DEC + DMC + MA (1:1:1:1) Ex. 7 0.75 EC + DEC + DMC + EA (1:1:1:1)Ex. 8 0.75 EC + DEC + DMC + EP (1:1:1:1) Ex. 9 0.75 EC + DEC + DMC + EB(1:1:1:1) Ex. 10 0.75 EC + DEC + DMC (1:1:1) Ex. 11 0.75 EC + DEC +DMC + EMC (1:1:1:1) Ex. 12 1.0 EC + DEC + DMC + EMC (1:1:1:2) Ex. 13 1.0EC + DEC + DMC + 0.01 M LiOCH₃ (1:1:1)

Ternary Solvent Systems

After evaluating the low temperature conductivity and assessing therelative stability of potential systems, a number of electrolytes wereselected for evaluation in lithium-ion experimental cells. The lowtemperature and cycle life performance of these cells was the basis forselecting six electrolytes for incorporation into prototype cells whichwere fabricated by Wilson Greatbatch Ltd. according to JPL design andpossessing electrolytes and electrode materials prepared at JPL. Theelectrolytes chosen for integration into the prototype cells consist ofthree ethylene carbonate-based solutions for use with graphite-typeanodes: 1.0 M LiPF₆ in EC+DMC (30:70), 1.0 M LiPF₆ in EC+DEC (30:70),and 1.0 M LiPF₆ in EC+DEC+DMC (1:1:1), and three propylenecarbonate-based electrolytes for use with coke-type anodes: 1.0 M LiPF₆in PC+DEC (50:50), 0.5 M LiPF, in PC+DEC (25:75), and 0.5 M LiPF₆ inPC+DME (50:50). Although a number of electrolyte solutions which consistof ternary and quaternary mixtures of carbonates with low viscosityadditives were investigated and observed to have higher low temperatureconductivity, the systems solely based on carbonate mixtures wereevaluated at the prototype cell level due to the superior roomtemperature cycle life and stability.

Electrolyte Conductivity

High EC or DMC content in electrolyte solutions generally results inpoor low temperature conductivity due to their high melting points andviscosities. Low temperature conductivity can be improved bysubstituting these solvents with carbonates of lower melting points,such as PC or DEC, or by the addition of a third component which canserve as a low viscosity additive. For example, electrolytes composed ofEC+DEC and EC+DEC+DMC both display higher conductivity at lowertemperature due to the use of DEC which has a lower melting point and alower viscosity, as shown in Table 4. It is evident that, as between thebinary and ternary EC-based electrolytes, the electrolyte consisting of1.0 M LiPF₆ in EC+DEC+DMC (1:1:1) displayed the highest conductivity attemperatures of −20° C. and lower. The improved conductivity of theternary system over the binary electrolytes at low temperatures is dueto the synergistic effect of having EC (which has good coordinatingability and a high dielectric constant), DEC (which acts to lower themelting point of the medium), and DMC (which helps to lower theviscosity of the system) present in the proper proportions. Goodconductivity of the ternary system is also a result of the fact that thelithium coordination complexes formed therein are more disordered,allowing higher ionic mobility.

TABLE 4 Electrolyte Conductivity (mS/cm) Sample Concentration SolventSystem −60° −40° −20° C. 0° C. R.T C. Ex. 1 1.0 M LiPF₆ EC + DMC (30:70)Fr. 1.9 7.1 12.2 C. Ex. 2 1.0 M LiPF₆ EC + DEC (30:70) Fr. 0.66 1.9 4.07.5 Ex. 1 1.0 M LipF₆ EC + DEC + DMC (1:1:1) 0.02 1.01 2.9 5.6 9.7 C.Ex. 4 0.5 M LiPF₆ PC + DEC (25:75) 0.28 0.97 2.1 3.7 5.7 C. Ex. 3 1.0 MLiPF₆ PC + DEC (50:50) 0.05 0.43 1.6 3.6 6.6 C. Ex. 5 0.5 M LiPF₆ PC +DME (50:50) 0.67 2.3  4.6 7.6 12.5

Nature of Electrolyte Interaction with Graphite Anode Electrodes

A number of cells were fabricated with EC-based electrolytes andevaluated in terms of the reversible and irreversible capacity uponcycling. As illustrated in Table 5, the cells containing electrolytesconsisting of 1.0M LiPF₆ in EC+DMC (3:7) display the lowest irreversiblecapacities of the series studied, whereas the cells with 1.0M LiPF₆ inEC+DEC (3:7) displayed the highest irreversible capacities. This trendsuggests that DMC has inherently greater stability compared with DECwhen placed in contact with lithiated carbon. As expected, cellscontaining an electrolyte consisting of 1.0M LiPF₆ in EC+DEC+DMC (1:1:1)displayed irreversible capacity values that are intermediary between theEC+DEC and EC+DMC-based cells. The data suggests that cells containingEC+DMC-based electrolytes display the least amount of electrolytedecomposition when in contact with graphite anodes under chargingconditions and therefore possess superior surface films. In contrast,the cells containing the 1.0M LiPF₆ in EC+DEC (3:7) electrolyte displayproperties indicative or greater electrolyte decomposition rates and/orthe formation of surface films without the desirable passivatingqualities. As expected, the cells containing an electrolyte consistingof 1.0M LiPF₆ in EC+DEC+DMC (1:1:1) displayed behavior consistent withthe fact that it is a mixture containing both DMC (less reactive) andDEC (more reactive).

TABLE 5 Reversible Irreversible Electrolyte Type Capacity CapacitySample Cell (All Contain 1.0 M LiPF₆) (mAh/g) (mAh/g) Ex. 1 A EC + DMC +DEC (1:1:1) 235.36 82.16 Ex. 1 B EC + DMC + DEC (1:1:1) 217.97 72.30 C.Ex. 2 C EC + DEC (30:70) 200.73 111.90 C. Ex. 2 D EC + DEC (30:70)215.31 139.49 C. Ex. 1 E EC + DMC (30:70) 221.86 47.18 C. Ex. 1 F EC +DMC (30:70) 261.57 48.91

Discharge Capacity at Different Rates and Temperatures

A number of cells were fabricated with each electrolyte and evaluated interms of the discharge characteristics and rate capability as a functionof temperature. A comparison of the low temperature dischargeperformance of graphite-based cells at −20° C., (FIGS. 2 and 3), showsthat the cells containing the electrolyte 1.0 M LiPF₆ in EC+DEC+DMC(1:1:1) delivered the highest capacity at −20° C., corresponding to over85% of the room temperature capacity. The discharge characteristicsobserved for cells at −20° C. correlate well with the conductivitytrend: EC+DEC+DMC >EC+DEC >EC+DMC. A significant aspect of thesemeasurements is that both the charge and discharge were performed at lowtemperature, in contrast to charging at ambient conditions and onlydischarging at low temperature. From the experimental results, it isevident that the incorporation of DEC into the solvents mixtures cangreatly improve the low temperature performance compared to thestate-of-the-art EC+DMC-based electrolyte systems.

The discharge capacities of the cells were evaluated over a temperaturerange of −40° to 23° C. and at a number of different charge anddischarge rates. As shown in FIG. 4, the discharge capacity of a cellcontaining 1.0 M LiPF₆ in EC+DEC+DMC (1:1:1) electrolyte was evaluatedover a large temperature range and was observed to deliver ˜60% of theroom temperature capacity at −30° C. at ˜C/20 rate. Graphite-basedprototype cells containing this ternary electrolyte displayed the bestlow temperature performance of the ternary EC-based systems studied.

As expected, the rate capability of the cells decreased with decreasingtemperature. However, as illustrated in FIG. 5, a cell containing 1.0MLiPF₆ in EC+DEC+DMC (1:1:1) and discharged at a rate of ˜C/5 at −20° C.was still capable of delivering 70% of the capacity displayed at a C/20rate.

When the discharge capacities of both the coke-type and graphite-typedesign prototype cells were evaluated at −30° C. under the same rateconditions (FIG. 6), the cell containing 1.0M LiPF₆ in EC+DMC+DEC(1:1:1) out-performed all other ternary systems dramatically. The highcapacity of this system is attributed to the high conductivity of theelectrolyte at low temperature and the inherently high capacity ofgraphite compared to coke-based systems.

The delivered discharge capacity can be increased greatly at lowtemperatures if low rates are used and the discharge cut-off voltage islowered below 3.0V. As shown in FIG. 7, when the discharge cut-offvoltage of a cell containing 1.0M LiPF₆ in EC+DMC+DEC (1:1:1) at −40° C.is lowered to 2.5V and the discharge rate decreased from 25 mA to 10 mA,the discharge capacity is increased by a factor of three to four,corresponding to approximately one fourth of the room temperaturecapacity at −40° C. Although it has been suggested that continueddischarge to low voltage may promote cell degradation mechanisms, theprocesses are likely to be less significant at lower temperatures due tochange in electrode potential as a function of temperature and slowerreaction kinetics.

When the cells were evaluated at extremely low temperatures (−58° C.),at very low rates (C/70 to C/150), and discharged to 2.0V, the cellcontaining 1.0M LiPF₆ in EC+DMC+DEC (1:1:1) electrolyte was observed todeliver >460 mah capacity, which corresponds to >90% of the roomtemperature capacity, as shown in FIG. 8.

Cycle Life Performance

In addition to evaluating the rate performance as a function oftemperature, the cycle life performance was assessed at both roomtemperature and at −20° C. Although the primary focus of the MarsExploration Program is to develop a rechargeable battery capable ofoperation at low temperature, room temperature cycling tests were deemednecessary to demonstrate the requisite stability during storage, launch,cruise or during daylight hours on Mars. The room temperature cyclingtests of a number of cells are currently in progress, and thegraphite-based AA size cells have completed >500 cycles to date, asshown in FIG. 9. The results demonstrate that the cells containing 1.0 MLiPF₆ in EC+DMC+DEC (1:1:1) electrolyte have excellent room temperaturecycle life with minimal capacity fade rate and high capacity.

Of the ternary organic solvent systems tested, the best low temperaturecycle life observed with graphite-based anodes was seen with 1.0M LiPF₆in EC+DMC+DEC (1:1:1) electrolyte, which initially delivered >84% of theroom temperature capacity at −20° C. and displayed >75 Wh/Kg at a C/10rate, as shown in FIG. 10. The cells containing 1.0M LiPF₆ in EC+DMC(30:70) and 1.0M LiPF₆ in EC+DEC (30:70) both showed inferior behaviorwith lower capacities and higher capacity fade rates.

Self-Discharge Characteristics

In order to determine the relative stability of the electrolytesolutions evaluated, the self-discharge behavior of prototypelithium-ion cells was investigated. As a general observation,lithium-ion cells having electrolyte systems that produce poorpassivating films on the electrode surfaces, or that participate indegradation mechanisms readily, often exhibit high discharge rates. Asillustrated in FIG. 11, the cells containing the EC-based electrolytesand graphite-based anodes displayed the smallest self-discharge rates,as evident from the cell voltage on OCV stand. When the EC-basedelectrolyte are compared, the electrolyte consisting of 1.0M LiPF₆ inEC+DMC+DEC (1:1:1) displayed a charge retention behavior comparable tothat of the EC+DMC electrolyte, which is widely used due to its provenstability.

After the prototype lithium-ion cells were placed in prolonged storagefor approximately six months at 0° C. in the fully charged state, theywere placed on charge to determine how much capacity had been lost dueto self-discharge mechanisms. As shown in FIG. 12, electrolyte-filledcells equipped with a graphite anode displayed the following trendexpressed in greatest resistance to self-discharge: 1.0M LiPF, in EC+DMC(30:70)>1.0M LiPF₆ in EC+DMC+DEC (1:1:1)>1.0M LiPF₆ in EC+DEC (30:70).These results suggest that the DMC-based electrolytes solutions displaymore favorable film formation characteristics and/or degradativereactivity.

Solvent Systems Containing Alkyl Esters

Detailed studies related to the surface film formation, low temperatureperformance and interfacial stability have been carried out to determinethe beneficial effects of ester additives to carbonate-basedelectrolytes in general and EC+DMC+DEC systems in particular.

Electrolyte Conductivity Measurements

A number of carbonate-based electrolytes containing low viscosity, lowmelting aliphatic ester additives were prepared and the conductivitymeasured over a temperature range of −60° C. to 25° C. The electrolytesassessed consisted of baseline formulations, 1.0M LiPF₆ in EC+DEC+DMC(1:1:1), to which the low viscosity co-solvents were added, including:methyl acetate (MA), ethyl acetate (EA), ethyl propionate (EP), andethyl butyrate (EB).

Of the electrolytes investigated, the formulations that displayed thehighest conductivity at low temperatures (FIG. 13) were ones containingthe lower molecular weight acetates and displayed the following trend:1.0M LiPF₆ EC +DEC +DMC +MA (11:1:1)>1.0M LiPF₆ EC+DEC+DMC+EA(1:1:1:1) >1.0M LiPF₆ EC+DEC+DMC+EP (1:1:1:1)>1.0M LiPF₆ EC+DEC+DMC+EB(1:1:1:1).

Lithium-Graphite Half Cell Studies

A number of lithium-graphite (KS 44 graphite) half-cells were fabricatedwith Li reference electrodes to study the effect of co-solvents upon thefilm formation characteristics on carbon electrodes (both graphite andMCMB-based materials). One purpose of these efforts was to determine theirreversible and reversible capacities of graphite electrodes in contactwith various electrolyte solutions. The lithium-graphite half-cells alsoserve as an additional screening test to identify the compatibility andstability of candidate electrolytes with carbonaceous electrodes. Theelectrolytes selected for evaluation included 0.75 M LiPF₆ dissolved in(a) EC+DEC+DMC+MA (1:1:1:1), (b) EC+DEC+DMC+EA (1:1:1:1), (c)EC+DEC+DMC+EP (1:1:1:1), (d) EC+DEC+DMC+EB (1:1:1:1), and (e) EC+DEC+DMC(1:1:1).

Charge/discharge Characteristics of Lithium-graphite Cells

One aspect of studying the charge/discharge characteristics of thelithium metal-carbon half-cells included the assessment of the observedirreversible and reversible capacities as a function of electrolytetype. These results are summarized below in terms of mAh/g of activecarbon used. When the group of electrolytes containing acetate additives(MA, EA, EP, EB) are considered, a correlation is observed with thehigher molecular weight additives resulting in higher reversiblecapacities after the fifth formation cycle. This is shown in FIG. 14.

The cells displayed the following trend in increasing reversiblecapacity: EA (214.2 mAh/g)>MA (236.5 mAh/g)>EB (309.46 mAh/g)>EP (340.75mAh/g). A similar type of trend was observed for the irreversiblecapacities, in that the higher molecular weight acetate-basedelectrolytes also tended to have higher irreversible capacities. Theseresults are summarized in the following table:

TABLE 6 Rev. Cap Irr. Cap Rev. Cap Irr. Cap Electrolyte-Type mAh/g mAh/gmAh/g mAh/g Sample (All Contain 0.75 M LiPF₆ (1st Cycle) (1st Cycle)(5th Cycle) (5th Cycle) Ex. 10 EC + DEC + DMC (1:1:1) 227.2 106.0 240.4127.1 Ex. 6 EC + DEC + DMC + MA (1:1:1:1) 201.5 36.9 236.5 56.9 Ex. 7EC + DEC + DMC + EA (1:1:1:1) 210.4 49.9 214.2 68.5 Ex. 8 EC + DEC +DMC + EP (1:1:1:1) 233.4 49.06 340.75 88.30 Ex. 9 EC + DEC + DMC + EB(1:1:1:1) 272.0 55.6 309.46 90.86

The charge/discharge characteristics of these cells was investigated asa function of temperature at varying rates. In general, the cellscontaining the higher molecular weight acetate (EP and EB)-basedelectrolytes displayed superior performance at low temperatures comparedwith the cells containing the lower molecular acetate (MA and EA)-basedelectrolytes. This is understood as being related to the nature of thesurface films formed on the carbon electrodes as the electrolyte type isvaried. Although the MA- and EA-containing electrolytes showed lowirreversible capacities (which is usually suggestive of an electrolytewith good passivating characteristics) they displayed large polarizationand charge transfer resistances (elaborated upon below). Thus, thekinetics of lithium intercalation and de-intercalation are not as faciledue to the impervious nature of surface films of the MA- andEA-containing electrolytes in contrast to the EP and EB-containingcells. At −20° C., when the de-intercalation process was studied at amodest rate (25 mA or ˜C/12) both the EB- and EP-electrolytes performedwell (>2/3 the room temperature capacity), with the EB-containingelectrolyte displaying performance superior to that of the baseline 1.0MLiPF₆ in EC+DEC+DMC (1:1:1) electrolyte at low temperature. This isshown in FIG. 15.

Electrochemical Characterization of the SEI Layer on Graphite Electrodesand the Effect of Electrolyte Composition

In addition to studying the charge/discharge characteristics of thesecells, AC impedance was used to probe the nature of the anodepassivating film. Measurements were conducted for each cell after theformation process (5 cycles), as well as, after the cells had beensubjected to cycling and characterized in terms of the self-dischargebehavior. When the group of acetate containing cells is compared interms of the charge transfer resistance, it was observed that the EB-and EP-containing cells displayed lower values than those observed withthe low molecular weight acetate derivatives. This is shown in FIG. 16.

The EB- and EP-containing cells also displayed less of an increase inthe R_(B) and R_(CT) values after cycling, compared with the MA- andEA-containing cells, suggesting that the surface films are moreinhibitive against further reaction of the electrolytes with the highermolecular weight additive.

AC impedance measurements were also taken at a number of temperatures(25, 0, −20, and −40° C.) to determine the effect of temperature uponthe film resistance of the samples. It was generally observed that thefilm resistance dramatically increased at low temperatures. Theseresults suggest that the nature of the SEI layer on the carbon electrodeplays a large role in determining the low temperature dischargeperformance in addition to the bulk resistivity of the electrolyte.Among the ester-based electrolytes, the EP- and EB-based electrolytesshowed more favorable behavior at −20° C. (FIG. 17) compared with theMA- and EA-based solutions, displaying significantly lower chargetransfer resistances.

DC micropolarization techniques were also employed to study the chargetransfer behavior of the passivating films on the graphite electrodes atvarious temperatures. The polarization resistance of the electrodes wascalculated from the slopes of the linear plots generated underpotentiodynamic conditions at scan rates of 0.02 mV/s (FIG. 18).

With the ester-containing electrolytes, the following trend ofincreasing polarization resistance is observed at room temperature:EC+DEC+DMC+EB (0.735 kOhms) <EC+DEC+DMC+EP (0.818 kOhms)<EC+DEC+DMC+MA(0.974 kOhms)<EC+DEC+DMC+EA (1.344 kOhms). A somewhat similar trend wasalso observed when investigated at low temperature (−20° C.):EC+DEC+DMC+EP (6.828 kOhms)<EC+DEC+DMC+EB (11.11 kOhms <EC+DEC+DMC+EA(16.60 kOhms). Thus, the electrolytes with the higher molecular weightester additives should perform better at low temperature compared withthe lower mol. weight ester due to the improved kinetics and smallerpolarization values. This trend correlates well with the data obtainedfrom the charge/discharge characterization of the cell at lowtemperature.

The limiting current densities were also determined for the lithiumdeintercalation from the graphite electrodes by conducting Tafelpolarization measurements to evaluate the rate capability of theseelectrodes. These measurements were conducted on the lithium-graphitecells at various temperatures (25, 0, −20, and −40° C.). The resultsobtained correlate well with the DC micro-polarization measurements inthat the cells possessing high polarization resistance, i.e. withelectrolytes possessing MA or EA, have lower diffusion limiting currents(measured at an over potential of 250 mV) compared with cells containingEP or EB as an electrolyte co-solvent.

Organic carbonate-based electrolytes containing low molecular weightester additives exhibit higher conductivity at low temperatures thanelectrolytes containing higher molecular weight ester additives.However, the interfacial stability with graphite during charge-dischargecycling is adversely affected such that the kinetics of lithiumintercalation are severely hindered with the low molecular weight estersystems. Electrolytes containing higher molecular weight esters, on theother hand, have adequate interfacial stability and enhanced electrolyteconductivity, both resulting in improved low temperature performance forlithium ion rechargeable cells.

In addition to the previously described cells, prototype AA-sizelithium-ion cells have been demonstrated to operate effectively attemperatures as low as −30 to −40° C. The electrolytes chosen forinvestigation in this series of cells include: 0.75M LiPF₆ inEC+DEC+DMC+MA (1:1:1:1:), 0.75M LiPF₆ in EC+DEC+DMC+GBL(gamma-butyrolactone) (1:1:1:1), 0.75M LiPF₆ in EC+DEC+DMC+EMC(1:1:1:1), 0.75M LiPF₆ in EC+DEC+DMC+EA (1:1:1:1), and 0.75M LiPF₆ inEC+DMC+MA (1:1:1). When the cells were charged at room temperature anddischarged at −40° C. (after approximately a four hour equilibrationperiod), excellent discharge capacity was delivered in some cases. Thecell having the ethyl acetate-containing electrolyte was observed todisplay the best performance, with >140 mAh being delivered at −40° C.,which corresponds to greater than 35% of the room temperature capacity.The cell having the 0.75M LiPF₆ in EC+DMC+MA (1:1:1) electrolyte alsodisplayed performance superior to that of the baseline electrolyte at−40° C. [1.0M LiPF₆ in EC+DEC+DMC (1:1:1)], delivering ˜110 mAhrcompared to only ˜75 mAhr for the baseline. When the cells wereevaluated at low rate (10 mA discharge current =C/40) at −40° C., thecell containing the 0.75M LiPF₆ in EC+DMC+MA (1:1:1) electrolyte wasobserved to deliver the highest discharge capacity (˜380 mAhr), followedby the cell containing the 0.75M LiPF₆ in EC+DEC+DEC+MA (1:1:1:1)electrolyte, both of which were superior to that of the baselineelectrolyte.

Asymmetric Carbonate-Based Systems

Lithium-Graphite Half Cell Studies

A number of lithium-graphite cells were fabricated to evaluate the lowtemperature electrolytes for lithium ion cells. The use of such threeelectrode half-cells facilitated the study of the effect of differentelectrolytes upon the film formation characteristics on carbonelectrodes (both graphite and MCMB-based materials). One purpose ofthese efforts was to determine the irreversible and reversiblecapacities of graphite electrodes in contact with various electrolytesolutions. The lithium-graphite half-cells also serve as an additionalscreening test to identify the compatibility and stability of candidateelectrolytes with carbonaceous electrodes. The following systems wereevaluated:

Sample Electrolyte Anode Ex. 10 0.75 M LiPF₆ EC + DEC + DMC (1:1:1:1)KS-44 Graphite Ex. 11 0.75 M LiPF₆ EC + DEC + DMC + EMC (1:1:1:1) KS-44Graphite Ex. 12 1.0 M LiPF₆ EC + DEC + DMC + EMC (1:1:1:2) KS-44Graphite C. Ex. 8 1.0 M LiPF₆ EC + DMC + EMC (1:1:1) KS-44 Graphite

The selection of these electrolytes is based upon the beneficialproperties of adding a low viscosity, low melting point solvent additive(ethyl methyl carbonate) to organic carbonate mixtures that have beenobserved to have the desirable stability and passivating qualities.Another underlying objective was to minimize the amount of EC used (andmaximize EMC content), without impairing salt dissolution, and to extendthe operation of these electrolytes to lower temperatures.

Charge/discharge Characteristics of Lithium-graphite Cells

One aspect of studying the charge/discharge characteristics of thelithium metal-carbon half-cells included the assessment of the observedirreversible and reversible capacities as a function of electrolytetype. Table 7 summarizes the results in terms of mAh/g of active carbonused. When the electrolytes containing different proportions of EMC as aco-solvent are compared, generally high reversible capacities (296-336mAh/g) and low irreversible capacities were observed after the fifthcycle. An attempt to quantify the effect of EMC incorporation in theelectrolyte upon the reversible and irreversible capacities iscomplicated by the effect of varying EC content in these formulations.

TABLE 7 Electrolyte-Type Rev. Cap Irr. Cap Rev. Cap Irr. Cap (A = 0.75 MLiPF₆) mAh/g mAh/g mAh/g mAh/g Sample (B = 1.0 M LiPF₆) (1st Cycle) (1stCycle) (5th Cycle) (5th Cycle) Ex. 10 A in EC + DEC + DMC (1:1:1) 227.2106.0 240.4 127.1 Ex. 6 A in EC + DEC (30:70) 266.1 106.6 275.4 136.9Ex. 7 A in EC + DMC (30:70) 302.0 94.3 312.6 122.9 C. Ex. 8 B in EC +DMC + EMC (1:1:1) 246.2 55.0 296.8 87.5 Ex. 11 A in EC + DEC + DMC + EMC(1:1:1:1) 292.4 47.6 317.0 87.5 Ex. 12 B in EC + DEC + DMC + EMC(1:1:1:2) 319.1 46.0 335.8 77.0

The charge/discharge characteristics of these cells was investigated asa function of temperature at varying rates. The cells containing 1.00MLiPF₆ in EC+DEC+DMC+EMC (1:1:1:2), 1.0M LiPF₆ in EC+DMC+EMC (1:1:1) and0.75M LiPF₆ in EC+DEC+DMC+EMC (1:1:1:1) also displayed large dischargecapacities at low temperature (FIG. 19).

Cells containing the above-described electrolytes displayed thefollowing trend in low temperature discharge capacity: 1.0M LiPF₆ inEC+DEC+DMC+EMC (1:1:1:2)>1.0M LIPF₆ in EC+DMC+EMC (1:1:1)>1.0M LiPF₆ inEC+DEC+DMC (1:1:1)>0.75M LiPF₆ in EC+DEC+DMC+EMC (1:1:1:1). It appearsthat the electrolyte formulation, 0.75M LiPF₆ in EC+DEC+DMC+EMC, can befurther improved in terms of low temperature performance by furtheroptimizing the concentration of the salt (from 1.0 to 1.25 M), sinceincreased salt concentration causes a depression in the freezing pointof the medium, without any prohibitive increase in the viscosity. In thecourse of the charge/discharge characterization, it was generallyobserved that the process of lithiation into carbon is less facile thanthe de-intercalation process (and is rate limiting), implying that thecharge time and rate is crucial to obtain effective performance at lowtemperature. FIG. 20 shows the formation cycles at room temperatures. Inall the cases, the capacity attained plateau values, suggesting theformation of a stable SEI on the anode.

Electrochemical Characterization of the SEI Laver on Graphite Electrodesand the Effect of Electrolyte Composition

In addition to studying the charge/discharge characteristics of thesecells, AC impedance was used to probe the nature of the anodepassivating film. Measurements were conducted for each cell after theformation process (5 cycles), as well as, after the cells had beensubjected to cycling and characterized in terms of the self-dischargebehavior. FIG. 21 illustrates the impedance behavior of the SEI-coveredgraphite anodes in different electrolytes at room temperature.

The resistance of the SEI in the EMC-containing solutions is comparableto each other and is less than the ternary mixture. After cycling, theresistance values were closer. AC impedance measurements were also takenat a number of temperatures (25, 0 −20, and −40° C.) to determine theimpact of temperature upon the film resistance of the samples. Asindicated in FIG. 22, the film resistance increases considerably at lowtemperatures. These results suggest that the nature of the SEI layer onthe carbon electrode plays a large role in determining the lowtemperature discharge performance in addition to the bulk resistivity ofthe electrolyte. Nevertheless, the values of SEI resistance arereasonably low in the EMC-containing solutions, even at −40° C.

DC micropolarization techniques were also employed to study the chargetransfer behavior of the passivating films on the graphite electrodes atvarious temperatures. The polarization resistance of the electrodes(FIG. 23) was calculated from the slopes of the linear plots generatedunder potentiodynamic conditions at scan rates of 0.02 mV/s.

The polarization resistance obtained (at room temperature and lowtemperatures) in the EMC-based solutions is comparable within eachsolution and with the ternary solution, suggesting that thecharacteristics of the SEI are similar. In other words, the kinetics forthe lithium intercalation being unaltered, the beneficial effects of theEMC additive (on electrolyte conductivity) could be realized. This isfurther supported by Tafel polarization data in various electrolytes andat various temperatures (25, 0, −20, and −40° C.).

Addition of EMC had no observable adverse effect either on the SEI or onthe kinetics.

Alkoxide Additives

Lithium methoxide was identified as an excellent electrolyte additive tofacilitate the in-situ ester exchange reaction in carbonate mixtures. Toinvestigate the viability of using lithium alkoxides as electrolyteadditives, a solution consisting of 1.0M LiPF₆ in EC+DEC+DMC (1:1:1 vol%) and approximately 0.01M LiOCH₃ was prepared and allowed to react forapproximately two weeks at room temperature. Under these conditions, asolution consisting of 1.0M LiPF₆ in EC+DEC+DMC+EMC will be formed dueto ester exchange reactions. The most probable composition of themixture described is EC+DEC+DMC+EMC (42.7:11.7:22.1:23.5 mol %) due tothe fact that the initial solution consisted of EC+DEC+DMC(42.7:23.5:33.8 mol %) and was assumed to reach a state of equilibrium.This solution was then added to a lithium-graphite half cell equippedwith a reference electrode, and the lithiumintercalation/deintercalation behavior in carbon was studied. Theelectrical performance cell was also evaluated in terms of the stabilityand low temperature performance.

Charge/discharge Characteristics of Lithium-graphite Cells

The charge/discharge characteristics of a number of cells containingcarbonate-based electrolytes was investigated as a function oftemperature at varying rates. Prior to these characterization cycles,the cells were cycled five times at room temperature as part of the cellformation process. As illustrated by the firstintercalation/de-intercalation cycle of the cell containing thecarbonate electrolyte with the lithium alkoxide additive (FIG. 24), highreversible lithium capacity was observed (322 mAh/g carbon) with KS-44graphite, with small irreversible capacity consumed in the first cycle(59 mAh/g). This reversible capacity is higher than that obtained withthe parent electrolyte (0.75M LiPF₆ in EC+DEC+DMC (1:1:1) and consistentwith the presence of EMC as supported by results obtained with theelectrolyte (0.75M LiPF₆ in EC+DEC+DMC+EMC (1:1:1:1). There was nodiminishment of reversibility observed over the first five formationcycles (an increase to 334 mAh/g reversible capacity was observed), asshown in FIG. 25. In addition, the irreversible capacity was observed tostay constant over the first five cycles, implying that the SEIformation process was essentially complete after the first cycle. Thissuggests that the nature of the surface films formed on the carbonsurface on the first cycle are very favorable and prevent furtherelectrolyte decomposition.

When electrolytes containing different proportions of EMC as aco-solvent were compared, generally high reversible capacities and lowirreversible capacities were observed (296-336 mAh/g) after the fifthcycle. An attempt to quantify the effect of EMC incorporation in theelectrolyte upon the reversible and irreversible capacities iscomplicated by the effect of varying EC content in these formulations.However, from Table 8 it is evident that the methoxide-containingelectrolyte behaved in a similar fashion to that of the EC+DEC+DMC+EMC(1:1:1:2) electrolyte, supporting the view that the incorporation of theelectrolyte additive did indeed result in the disproportionation of thestarting symmetric carbonates. A significant result from thesecomparisons is that the methoxide-containing electrolyte resulted in thecell with the least amount of irreversible capacity loss and nearly thehighest reversible capacity obtained.

TABLE 8 Electrolyte-Type (A = 0.75 M LiPF₆) Rev. Cap Irr. Cap Rev. CapIrr. Cap Sample (B = 1.0 M LiPF₆) (1st Cycle) (1st Cycle) (5th Cycle)(5th Cycle) Ex. 1 B in EC + DEC + DMC (1:1:1) 227.2 106.0 240.4 127.1 C.Ex. 1 B in EC + DMC (30:70) 302.0 94.3 312.6 122.9 Ex. 11 A in EC + EC +DMC + EMC (1:1:1:1) 292.4 47.6 317.0 87.5 Ex. 12 B in EC + DEC + DMC +EMC (1:1:1:2) 319.1 46.0 335.87 77.0 C. Ex. 8 B in EC + DMC + EMC(1:1:1) 246.22 54.97 296.76 87.51 Ex. 13 B in EC + DEC + DMC + (1:1:1)322.4 58.5 334.1 55.8 + 1.0 M LiOCH₃

Low Temperature Performance

The charge/discharge characteristics of these cells was investigatedunder similar conditions as a function of temperature. As shown in FIG.26, when the methoxide-containing electrolyte was investigated, morethan 95% of the room temperature lithium capacity was realized at −20°C. (>300 mAh/g) at a˜C/12 rate. This performance is superior to that ofthe baseline electrolyte, 1.0M LiPF₆ IN EC+DEC+DMC (1:1:1), as well as anumber of other carbonate mixtures investigated, as shown in FIG. 27.The observed graphite discharge capacity of the 1.0M LiPF₆ in EC+DEC+DMC(1:1:1)+LiOCH₃ was very similar to the results obtained with the 1.0MLiPF₆ in EC+DEC+DMC+EMC (1:1:1:2) electrolyte. This further supports thecontention that the base-catalyzed exchange reaction was allowed toreach equilibrium concentrations, resulting in the introduction of EMCand the formation of a quaternary solvent mixture similar to the oneinvestigated.

In addition to evaluating the cells at −20° C., the discharge capacitiesof the graphite electrodes was investigated at −40° C. As shown in FIG.28, the methoxide-containing electrolyte resulted in the highestcapacity (room temperature charge) delivering ˜150 mAh/g.

Self-Discharge Behavior

A significant portion of the self-discharge in a Li ion cell is believedto occur at the carbon anode, implying that it is a strong function ofthe nature of the electrolyte solution due to its role of formingprotective surface films. Thus, the extent of self-discharge (or extentof capacity retention) can reflect the nature of the SEI film on thecarbon electrode. In an attempt to further elucidate the mechanism ofself-discharge, the cells containing various carbonate-basedelectrolytes were fully charged and left under open circuit conditions.By monitoring the cell voltage, some indication of the electrode stateof charge can be discerned (although it is difficult to quantify due tothe non-linear response). As shown in FIG. 29, the methoxide-containingelectrolyte displayed favorable characteristics (small voltage decay)which was similar to the 1.0M LiPF₆ in EC+DEC+DMC+EMC (1:1:1:2)electrolyte.

Electrochemical Characterization of the SEI Layer on Graphite Electrodesand the Effect of Electrolyte Composition

In addition to studying the charge/discharge characteristics of thesecells, a.c. impedance was used to probe the nature of theanode-passivating film. Measurements were conducted for each cell afterthe formation process (5 cycles), as well as after the cells had beensubjected to cycling and characterized in terms of the self-dischargebehavior. As shown in FIG. 30, when the ac impedance measurements wereconducted after the formation cycle, the methoxide-containingelectrolyte displayed very favorable characteristics, with low seriesand film resistance being analogous to that of the cell containing theEC+DEC+DMC (1:1:1:1) electrolyte.

The methoxide-containing cell also displayed less of an increase in theRB and RT values after cycling, as shown in FIG. 31, compared with othercarbonate-containing cells, suggesting that the surface films are moreinhibitive against further reaction of the electrolytes with theincorporation of the additive. This aspect is especially noteworthysince the low temperature performance of lithium ion cells containingvarious low temperature electrolytes has been observed to degrade withcycling, implying that the SEI layer on the carbon electrode becomesmore resistive with prolonged operation. The behavior observed with theincorporation of lithium methoxide suggests that SEI formation isespecially complete after the first cycle, and little furtherelectrolyte decomposition occurs preventing the progressive increase inthe resistivity of the carbon surface films.

AC impedance measurements were also taken at a number of temperatures(25, 0, −20, and −40° C.) to determine the impact of temperature uponthe film resistance of the samples. It was generally observed that thefilm resistance dramatically increased upon going to lower temperatures.These results suggest that the nature of the SEI layer on the carbonelectrode plays a large role in determining the low temperaturedischarge performance in addition to the bulk resistivity of theelectrolyte. When a number of cells containing carbonate-based solutionswere compared at −20° C., the cells containing the methoxide additivedisplayed better performance characteristics than cells containing onlythe carbonates, including LiPF₆ in EC+DMC+EMC (1:1:1), EC+DEC+DMC+EMC(1:1:1:1) and EC+DEC+DMC+EMC (1:1:1:2). This is shown in FIG. 32.

DC micropolarization techniques were also employed to study the chargetransfer behavior of the passivating films on the graphite electrodes atvarious temperatures. The polarization resistance of the electrodes wascalculated from the slopes of the linear plots generated underpotentiodynamic conditions at scan rates of 0.02 mV/s (FIG. 33). Whenthe carbonate-based electrolytes were compared at room temperature, itwas generally observed that, with increasing EMC content, thepolarization resistance of the electrode increased, with themethoxide-containing electrolyte exhibiting the greatest polarization:EC+DEC+DMC (1:1:1) (0.760 kOhms)<EC+DEC+DMC+EMC (1:1:1:1) (0.822kOhms)<EC+DEC+DMC+EMC (1:1:1:2) (0.876 kOhms)<EC+DMC+EMC (1:1:1) (0.994kOhms)<EC+DMC+EMC (1:1:1)+LiOMe (1.014 kOhms). A different trend wasobserved, however, when the cells were evaluated at −20° C., with themethoxide-containing electrolyte displaying the lowest amount ofpolarization when contrasted with the other electrolytes: EC+DMC+EMC(1:1:1)+LiOMe (7.576 kOhms)<EC+DEC+DMC (7.876 kOhms)<EC+DMC+EMC (1:1:1)(9.90 kOhms)<EC+DEC+DMC+EMC (1:1:1:2) (12.99 kOhms)<EC+DMC+DEC+EMC(1:1:1:1) (13.16 kOhms). These results suggest that the methoxide-containing electrolyte produces carbon filmed electrodes which displayfacile lithium intercalation/de-intercalation kinetics at lowtemperature (similar to the favorable characteristics observed withEC+DEC+DMC) while also possessing the more favorable physical propertiesand high reversible capacities associated with EMC-containingelectrolytes.

The limiting current densities were also determined for the lithiumdeintercalation process from the graphite electrodes by conducting Tafelpolarization measurements to evaluate the rate capability of theelectrodes in contact with the various electrolytes. These measurementswere conducted on the lithium-graphite cells at various temperatures(25, 0, −20, and −40° C.). When the effect of various carbonate-basedelectrolyte types upon the polarization characteristics of carbonelectrodes are studied at room temperature (FIG. 34), the lithiummethoxide- containing electrolyte displayed behavior closely resemblingthat of the EC+DEC+DMC+EMC (1:1:1:2) electrolyte, being consistent withthe in situ formation of EMC. When the cells were evaluated at −20° C.C, (FIG. 35), the electrolyte containing the lithium methoxide additiveresulted in the least amount of electrode polarization and the highestlimiting current densities, implying facile lithiumintercalation/de-intercalation kinetics. These results are consistentwith the trend observed when the cells were evaluated using linearpolarization techniques, as well as with the cell charge/dischargecharacteristics at low temperature.

Other Approaches for Improved Low Temperature Performance of Lithium IonCells

From the foregoing, it will be appreciated that a number of electrolyteformulations fit within this approach to improving the low temperatureperformance of lithium ion battery electrolytes. Non-limiting example ofsuch formulations include:

(a) LiPF₆ in EC+DEC+DMC (1:1:1 vol %)+LiOCH₃, (b) LiPF₆ in EC+DEC+DMC(1:1:1 mol %)+0.01 M LiOCH₃, (c) LiPF₆ in EC+DPC+DMC (1:1:1 mol%)+LiOCH₃, (d) LiPF₆ in EC+DBC+DMC (1:1:1 mol%)+0.01M LiOCH₃, (e) LiPF₆in EC+EMC (1:3)+LiOCH₃, (f) LiPF₆ in PC+EMC (1:3)+LiOCH₃, (g) LiPF₆ inPC+DEC+DMC (1:1:1)+LiOCH₃, (h) LiPF₆ in EC+DEC+DPC+LiOCH₃ (i) LiPF₆ inEC+DEC+DBC+LiOCH₃, (j) LiPF₆ in EC+DMC+DEC+DBC, (k) LiPF₆ inEC+DPC+DBC+LiOCH₃ (1) ester-containing electrolytes (MA, EA, PA, EP, EB,etc.), (m) ester+alkyl carbonate-containing electrolyte mixtures (MA,EA, PA, EP, EB+DEC, DMC, DPC, DBC and EMC), (n)perfluro-ester-containing electrolytes (F-MA, F-EA, F-PA, F-EP, andF-EB), (o) perfluro-ester+alkyl carbonate-containing electrolytemixtures (F-MA, F-EA, F-PA, F-EP, and F-EB+DEC, DMC, DPC, DBC and EMC),and (p) ester+perfluro-alkyl carbonate-containing electrolyte mixtures(MA, EA, PA, EP, EB+F-DEC, F-DMC, F-DPC, F-DBC and F-EMC). “DBC” is anabbreviation for dibutyl carbonate.

Many permutations of the electrolytes described herein can be used. Theamount, concentration, relative proportions, and identity of thesolvents and salt can be modified within the broad confines of theinventions. Lithium alkoxides besides broad confines of the inventions.Lithium alkoxides besides lithium methoxide can serve as the reactioncatalyst species, including LiOCH₂CH₃, LiOCH₂CH₂CH₃, LiOCH₂CH₂CH₂CH₃,LiOCH(CH₃)₂, LiOC(CH₃)₃, LiOCF₃, and LiOCF₂CF₃. LiOCOOR and LiOCOR(where R=—CH₃, —CH₂CH₃, —CH₂CH₂CH₂CH₃, —CH(CH₃)₂, —C(CH₃)₃, —CF₃,—CF₂CF₃, etc.) are also acceptable.

The beneficial properties of asymmetric carbonates in terms of lowtemperature performance of lithium-ion electrolytes (low melting point,compatibility with the cell, good film formation characteristics, highlithium reversibility), can be realized with an in-situ formationprocess based on the use of compounds of the formula LiOX, preferably incatalytic amounts, to mixtures of precursor electrolyte formulations. Inaddition to possessing desirable physical properties, such as low mp,the multi-component character of the resulting quaternary solution(EC+DEC+DMC+EMC) allows for increasing disorder in the lithium solventcoordination sphere and generally results in higher conducting solutionsat low temperature due to the lack of “structuredness”.

From lithium-graphite half-cells studies, it was demonstrated that theaddition of lithium methoxide to a ternary mixture of carbonatesresulting in improved low temperature characteristics. In addition topossessing the desired low temperature characteristics, behaving in asimilar fashion to that of the 1.0M LiPF₆ in EC+DEC+DMC+EMC (1:1:1:2)electrolyte, the methoxide-containing electrolyte should posses superiorhigh temperature performance in that it possesses a higher EC content(33 vs. 20 vol % EC), which has been observed to result in lowercapacity fade during high temperature cycling (25-50° C.).

This invention in its broader aspects is not limited to the specificdetails shown and described herein. Departures may be made from suchdetails without departing from the principles of the invention andwithout sacrificing its chief advantages. For example, the improvedorganic solvent systems and electrolytes described herein are alsosuitable for use in polymer matrix and gel electrolyte electrochemicalcells, including polyvinylidene difluoride, polyacrylonitrile, otheracrylic polymers, and other host matrices known in the art.

Throughout the text and the claims, use of the word “about” in relationto a range of numbers is intended to modify both the high and low valuesrecited.

What is claimed is:
 1. An organic solvent system for an electrochemicalcell, comprising: a 1:1:1 equal volume mixture of ethylene carbonate,dimethyl carbonate, and diethyl carbonate.
 2. An electrolyte comprisinga lithium salt dispersed in an organic solvent system as recited inclaim
 1. 3. An electrolyte as recited in claim 2, wherein the lithiumsalt is LiPF₆.
 4. An electrolyte as recited in claim 2, wherein the saltis present in a concentration of from about 0.5M to 1.5M.
 5. Anelectrochemical cell, comprising: an anode; a cathode; and interspersedtherebetween an electrolyte comprising a lithium salt dispersed in anorganic solvent system as recited in claim
 2. 6. An organic solventsystem for an electrochemical cell, comprising: a mixture of ethylenecarbonate, dimethyl carbonate, diethyl carbonate, and a fluoroalkylester.
 7. An organic solvent for an electrochemical cell, comprising: amixture of ethylene carbonate, dimethyl carbonate, diethyl carbonate anda compound of the formula LiOX, where X is R, COOR, or COR, where R isperfluoroalkyl.
 8. An organic solvent system for an electrochemicalcell, comprising: a mixture of ethylene carbonate, dimethyl carbonate,diethyl carbonate, and a compound of the formula LiOR, where R isfluoroalkyl.
 9. An organic solvent system as recited in claim 8, whereinR is perfluoroalkyl.
 10. An organic solvent system as recited in claim8, wherein the compound of the formula LiOR is present in aconcentration of about 0.0001M to 0.1M.